Clean Coin by lopp: https://github.com/jlopp/cleancoin

"The most environmentally friendly version of bitcoin."

CleanCoin is built on top of Bitcoin Core. The difference between the two is the consensus algorithm: CleanCoin uses Proof of Stake consensus, whilst Bitcoin uses Proof of Work. Using Proof of Stake as a consensus algorithm is allowing it not only to scale better and be orders of magnitude more efficient in terms of power consumption, but it is also lowering the entry barrier for contributing to the creation of new blocks.